Tuning indicator circuit



Patented June 11, 1940 UNITED STATES TUNING INDICATOR CIRCUIT Fred B. Stone, Haddon Heights, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April 30, 1938, Serial No. 205,205

12 Claims.

The present invention relates to tuning indicator circuits for radio receiving systems, and more particularly it relates to a tuning indicator circuit embodying an. indicator tube of the type known commercially as an RCA6E5 called Magic Eye. This tube, as is well known, is of the cathode ray type having a cathode and atarget electrode with a control electrode interposed between the cathode and the target to cast a shadow on the target with an increasingly wider angle as the control electrode becomes less positive with respect to the cathode. The tube also includes a triode amplifier having an output resistor coupled to the control eletcrode to value the D. C. potential thereon, thereby to control the shadow on the screen.

The amplifier portion of the tube is necessary in order to permit it to be coupled to a source of biasing potential responsive to signal strength such as the output of a second detector, as shown, for example, in my copending application Serial No. 7,374, filed March 28, 1936, for improvement n Resonance tuning indicator system, assigned to the same assignee as this application. The presence of the amplifier within the tuning indicator tube tends to make the tube relatively large and not readily adapted to smaller radio receivers and the like and requires that it be connected to the second detector or other source of variable D. C. bias responsive to signal strength variations, which is often not convenient or desirable.

Accordingly, it is an object of the present in vention, to provide an improved tuning indicator circuit involving an indicator tube of the elec- 9 tron beamor cathode ray type wherein the amplifier within the tube may be eliminated, thereby permitting a tube of smaller size to be employed, and further eliminating the necessity for any external amplifier for the tuning indicator tube.

It is also a further object of the invention to provide a tuning indicator circuit embodying an electron beam indicator tube, the indicator elements of which may be actuated without pro riding a separate amplifier or amplifier tube and an indicator circuit therefor, including a portion of an intermediate frequency amplifier, whereby response to signal strength variations may be obtained through the medium of the A. V. C. means connected with such amplifiers in a receiving system of a superheterodyne type.

It is a still further object of thepresent invention to provide a tuning indicator circuit embodying a tuning indicator tube of the electron beam type which may be operated directly from one of the intermediate frequency amplifier tubes and which permits a relatively wide angle of change in the shadow arrangement between resonance and non-resonance in the tuning adjustment of the receiver.

The invention will, however, be better understood from the following description, when considered in connection with the accompanying drawing, and its scopewill be pointed out in the appended claims.

Referring to the drawing, inwhich the figure shows a schematic circuit diagram of a tuning indicator circuit embodying the invention, 5 is an intermediate frequency amplifier tube of the screen grid type having a control grid 6, a screen grid I, a cathode 8, an output anode 9 and a suppressor grid I0.

Intermediate frequency signals from a preceding amplifier or first detector (not shown) are applied to the control grid 6 through a tuned interstagecoupling transformer I I, the secondary I2 of which is connected through its high potential terminal I3 to the control grid 6. The low potential terminal I4 of the secondary I2 is connected with an A. V. C. supply lead I5 for the receiving system, which receives a controlling A. V. C. potential from the second detector indicated at I6, across the output resistor I! thereof through a filter resistor I8. The detector is coupled to the amplifier 5 through an interstage coupling transformer I9 and is of the diode rectifier type having a cathode connected to ground as indicated at 2|, whereby the bias supply circuit from the resistor I1 is completed to the cathode 8 and other amplifier cathodes (not shown) directly through a ground connection 22 for said cathode.

For the tuning indicator system under consideration, this type of control is preferable in that it permits all of the cathodes of the controlled tubes to be connected to ground and the controlled grids to be connected with the common A. V. C. supply lead l5.

In modern radio receiving systems, the amplifier tube, such as the tube 5, is subjected to a relatively wide range of A. V. C. potentials extending in some cases from 0 to 40 volts. It has been found that with this range of control a more gradual cut oil and consequently lower distortion in the receiver may be realized, and a source of controlling potential for the indicator tube may be obtained if a relatively high resistance indicated at 23, is inserted in series with the screen grid 1 between it and the positive B supply lead indicated at 24, so that, with no signals, the screen current is sufiicient to drop the voltage at the screen to substantially 90 volts with a voltage of 250 at the lead 24. This may require a coupling resistor 23 of 100,000 ohms resistance. The range of control potential variation available across said resistor is sufiicient without amplification, to operate the control electrode of an electron beam indicator tube such as a tube indicated at 25. This tube is greatly simplified with respect to the well-known electron beam tube including the amplifier, such as the RCAGES, since it may include only a target electrode 26, a cathode 21 extending through the target and a control electrode 28, together with the heater 29 for the cathode.

The control electrode 28 extends through the target and casts a shadow in the electron beam which extends radially from the cathode to the target 26. The control electrode is connected to the screen grid end 30 of the screen grid series resistor 23 through a connection lead indicated at 3!. The target electrode 26 is connected directly to the +13 supply lead 24 through a lead 32 and the cathode is connected through a biasing resistor 33 to the negative 13 supply or chassis ground indicated at 34.

While the amplifier tube 5 is preferably an intermediate frequency amplifier tube, it may be any screen grid tube in a receiving system which is subjected to automatic volume control bias: and whose screen grid current decreases with increased negative bias on its control grid.

As the signal strength increases, an increasing negative bias from. the negative terminal 35 of the output resistor I! is applied between the oathode 8 and the control grid 6 causing the screen current to the screen 1 to decrease, thereby causing a decrease in the potential drop through the resistor 23, therefore, causing the potential of the control electrode 28 to approach that of the target 26. In the presence of strong signals, the screen current may be reduced to such a value that the screen grid is substantially at anode po tential, whereby the difference in potential. be tween the control electrode 28 and the target Z6 is substantially zero and the Width of the shadow may become relatively small.

As the signal strength decreases, the potential drop across the resistor 23 increases accordingly, causing the electrode 28 to become more negative with respect to the target 26 and, therefore, to more widely deflect the electron beam on the target 26. This causes the shadow angle to in crease proportionately. The tuning indicator tube indicates resonance in this circuit when adjusted to minimum shadow angle.

It has been found that the range of increase and decrease of the shadow angle may be materially increased through the use of a series cathode or bias resistor 33 in circuit with the tuning indicator tube, and this may be of such a resistance value that the cathode 2? becomes more positive or above ground by substantially volts. The bias resistor 33 may have a resistance value of 50,000 ohms and has been found to approximately double the shadow angle range on the type of tuning indicator tube indicated in the drawing. w

The tuning indicator circuit described has the advantage that no additional amplifier either within the tuning indicator tube or outside the tube is required for its operation in a receiving system. This not only permits greatly reducing the size of the tube and its cost, but also simplifies the circuit required to operate it. Furthermore, by utilizing one of the RF or IF amplifier tubes in a receiving system under control of suitable A. V. C. means, the variable D. C. bias potential required by the control electrode of the tube may be obtained by a high resistance series resistor in the screen circuit of the amplifier which is under A. V. C. control, to give an extended cutoff characteristic to the amplifier and a wide range of potential change in response to the A. V. C. potential for controlling the indicator tube directly without the intermediary of additional amplifiers.

This system also has the further advantage that the tuning indicator is not connected with any signal conveying circuit and that the screen grid series resistor may provide the variable potential for the control electrode thereby simplifying the circuit and eliminating the necessity for shielding of extended control leads for the indicator tube as has been required heretofore.

I claim as my invention:

1. In a radio receiving system, a tuning indicator circuit comprising a screen grid signal amplifier tube having a control grid, means for applying to said grid A. V. C. potentials variable in accordance with signal strength variations, a screen grid potential supply circuit for said tube including a series high resistor in the positive side thereof, a tuning indicator tube of the electron beam type having a target electrode, a cathode and a control electrode, means for connecting said cathode with a negative side of said supply circuit, and means for applying potential from said screen grid resistor between said control electrode and said target electrode.

In a radio receiving system, the combination with a signal amplifier tube having a control grid, a screen grid. and a cathode, means for applying a variable biasing potential between said control grid and cathode responsive to signal strength variations, means providing a relatively high resistance in circuit with said screen grid, an indicator tube having a positive target electrode, a more negative cathode electrode, and a control electrode interposed between said cathode and said target electrode, said control electrode being connected with the screen grid, means for applying a positive operating potential to said screen grid and said target electrode having a negative circuit connection with said tuning indicator tube cathode, and a relatively high resistance in said last-namecl connection.

3. In a radio receiving system, a tuning indi cator circuit including a tuning indicator tube having a positive target electrode, a cathode and a control electrode, means for applying operating anode potentials to said tube including a negative circuit connection with said cathode and a positive connection with said target electrode, a screen grid amplifier tube in said receiving system having a screen grid circuit including a relatively high series resistor connected between said target electrode and said control electrode, and means including a diode signal rectifier coupled to said amplifier for applying a controlling potential to said amplifier in response to signal strength variations.

4. In a radio receiving system, a tuning indicator circuit including a tuning indicator tube having a positive target electrode, a cathode and a control electrode, means for applying operating anode potentials to said tube including a negative supply connection with said cathode and a positive supply connection with said target electrode, a screen grid amplifier tube in said receiving system having a screen grid circuit including a relatively high series resistor connected between said target electrode and said control electrode,

and means including a diode signal rectifier coupled to said amplifier for applying a controlling potential to said amplifier in response to signal strength variations, said amplifier and rectifier each having a negative supply connection with said first named cathode.

5. The combination with a tuning indicator tube of the cathode ray type having a target electrode, a cathode and a control electrode responsive to a variable D. C. bias potential to vary the pattern of the electron beam indication on said target electrode, of means for applying said potential thereto comprising a screen grid amplifier tube, the screen grid current of which decreases with increased negative control grid bias, and a high resistance series control resistor for the screen current thereof connected between the target electrode and control electrode of said indicator tube.

6. The combination with a tuning indicator tube of the cathode ray type having a target electrode, a cathode and a control electrode responsive to a variable 'D. C. bias potential to vary the pattern of the electron beam indication on said target electrode, of means for applying said potential thereto comprising a screen grid amplifier tube, the screen grid current of which decreases with increased nega tive control grid bias, a high resistance series control resistor for the screen current thereof connected between the target electrode and control electrode of said indicator tube, and a resistor in series with the cathode of said indicator tube for increasing the range of pattern variation of the electron beam indication.

'7. The combination with a tuning indicator tube of the cathode ray type having a target electrode, a cathode and a control electrode responsive to a variable D. C. bias potential to vary the pattern of the electron beam indication on said target electrode, of means for applying said potential thereto comprising a screen grid amplifier tube, the screen grid current of which decreases with increased negative control grid bias, a high resistance series control resistor for the screen current thereof connected between the target electrode and control electrode of said indicator tube, means for placing a positive potential on the cathode of said indicator tube for increasing the range of tuning indication thereof, comprising a series cathode resistor in circuit with the cathode thereof, means for applying received signals to said amplifier tube, and means including a signal wave rectifier for controlling the negative control grid bias for said amplifier in accordance with signal strength variations.

8. In a superheterodyne receiver, the com bination with a screen grid intermediate frequency amplifier tube responsive to the A. V. C. means, of a screen grid series resistor for said amplifier tube having a relatively high resistance value such that it controls the gain response characteristic of the amplifier, and a tuning indicator tube having a positive target electrode connected with the positive end of said resistor, a control electrode connected with the negative or screen grid end of said resistor and a cathode provided with chassis ground connection, said last named connection including a series high resistor having a resistance value such that it raises the cathode of said indicator tube above chassis or ground by a predetermined amount whereby the indicating range of said tube is increased.

9. The combination with a tuning indicator tube having a cathode, a target electrode for receiving an electron beam from the cathode and a control electrode interposed between said cathode and target electrode, of means for applying a variable D. C. bias potential to said control electrode with respect to the target electrode to cause the potential of the control electrode to approach that of the target in response to strong signals, said means comprising a screen grid signal amplifier tube, the screen grid current of which decreases with increased negative biasing potential on said tube, means for causing said negative biasing potential to increase in response to increased signal strength, and a series resistor controlling the screen grid current to said amplifier tube and the amplification characteristic thereof connected between said target electrode and said control electrode, whereby the control electrode is more negative than the target electrode in the absence of signals.

10. In a radio signal receiving system, the combination with a screen grid signal amplifier tube having a control electrode, a cathode, a screen grid and an output anode, of means for applying a negative biasing potential to said control grid which increases with increased signal strength, a high resistance element in circuit with said screen grid providing a relatively wide range of potential change on said grid in response to said bias change, the screen current decreasing with increased negative bias, a tuning indicator tube having a cathode, a target electrode and a control electrode, said target electrode being connected with the positive end of said resistance element and the control electrode being connected with the screen or negative end of said resistance element, an anode potential supply circuit for said system having a positive lead connected with said target and having a negative connection with the cathode of said amplifier and indicator tube, the connection to the indicator tube including a resistance element providing a potential drop whereby the cathode of the indicator tube is more positive than the cathode of the amplifier tube.

11. In a radio receiving system, a tuning indicator circuit comprising a screen grid signal amplifier having an operating characteristic such that the screen current thereof decreases with increased negative bias on the control electrode thereof, a positive anode potential supply lead for said system, a resistor connected be tween said lead and said tube for controlling the flow of screen grid current to said tube and having a resistance value such that the tube is provided with an extended operating cut-off range, a tuning indicator tube having a cathode, a positive target electrode connected with said positive supply lead and a control electrode interposed between the cathode and the target electrode to cast a shadow in the electron beam on the target with an increasing angle as the'control electron becomes more negative with respect to tion on said target electrode, of means for applying said potential thereto comprising a screen grid amplifier tube the screen current of which varies with control bias, and a high resistance series control resistor for the screen current thereof connected between the target electrode and control electrode of said indicator tube.

FRED B. STONE. 

